Process of digesting wood or other ligno-cellulose materials



March 5, 1940- G. v. PALMROsE 2,192,239

PROCESS OF DIGESTING WOOD OR OTHER LIGNO-CELLULOSE MATERIALS Filed Dec. '#1937 ATTORNEY Patented Mar. 5, 1940 UNITED STATES PROCESS George Victor F DIGESTING WOOD OR OTHER GNO-CELLULOSE MATERIALS Palmrose, Longview, Wash., as-

signor to Weyerhaeuser Timber Company, Tacoma, Wash., a corporation of Washington Application December 7, 1937, Serial No. 178,467

4 Claims.

This invention relates to the digestion of wood and other cellulose materials to obtain cellulose fibres, and particularly to an improved sulphite -process in which material savings can be effected in the recovery of cellulose. y

The sulphiteprocess as heretofore conducted involves digestion of wood chips or like material with abisulphite of an alkali or alkaline earth metal (combined sulphur dioxide), with the addition of free sulphur dioxide. Heat is supplied at a carefully regulated rate to raise the temperature slowly during the digestion. It has been assumed that successful cooking depends upon what is termed mechanical penetration of the chips with the sulphite solution before the temperature reaches 110 C. This temperature is considered to be a critical temperature for penetration of the combined sulphur dioxide. Approximately three hours are allowed for such peneration. if a temperature of 110 is obtained sooner, it has been found that a portion of the chips, instead of being pulped, will appear in the blow pit in their original form, though darker in color. Some of the chips vmay be partly pulped at the ends, but hard and dark in the center. This phenomenonis called burningf and is carefully avoided in common practice.

To permit mechanical penetration of the chips, it has been the practice to employ suicient liquor to cover the chips in the digester. This requires about 60 gallons of liquor per hundredweight of dry wood, which, if we assume the liquor to have practically the same density as water, makes the ratio 5 pounds of liquor to 1 pound of dry wood.

As the result of this practice, the liquor, after digestion, is quite dilute, and the recovery of by-products is costly. Furthermore, there are substantial heat losses in the digestion, and other disadvantages which are overcome by the process as hereinafter described.

It is the object of the invention to provide an improved sulphite process in which a much lower ratio of sulphite liquor to dry wood is employed. The amount of combined sulphur dioxide supplied is that required to effect the desired pulping, and consequently the concentration of the sulphite liquor is materially increased.

Another object of the invention is the provision of a process in which waste liquor resulting from digestion is highly concentrated so that byproducts are readily recovered at lower cost.

A further object of the invention isthe provision of a process wherein the disadvantages of l the well known sulphite process are avoided and economy of operation is substantially improved.

Other objects and advantages of the invention will be apparent as it is better understood by reference-to the following specification and accompanying drawing, in which the gure is a diagrammatic illustration of a digester adapted to be used in carrying be understood that the out the process. It will drawing is merely illustrative and that other mechanical devices may be utilized for the purpose of the invention.

It has been the commonly accepted opinion that a substantial period of time must elapse during the preliminary heating of the digester in the sulphite process to permit mechanical penetration of the chips by the sulphite solution. Formulae have been developed and applied to indicate the time required, based on the well known principle that resistance to penetration is due to a series of capillary tubes and air pockets.

I have determined by experiment that wood, when thoroughly saturated with water or liquor, will contain approximately 1.6 pounds of water per pound of oven dry wood. Western hemlock, as used in commercial practice, is about one-half water. Consequently, only 0.6 pound of the ilquor used in the ordinary sulphite process could. enter each pound of chips by mechanical penetration, and the 4.4 pounds of liquor outside the chips would contain the bulk of the-combined sulphur dioxide necessary to properly cook the wood. Obviously there is something wrong with the theory of mechanical penetration.

Furthermore, it is evident that "burning of the chips is not the result of failure of mechanical penetration. Burning" results from the fact that some of the chips have become deiicient in combined sulphur dioxide. Polymerization of the lignin, as a result of the presence of free lignin sulphonic acid, takes place. If mechanical penetration must be depended upon, it is evident from the foregoing example of the effect upon wet chips that burning could not be avoided in treating such material.

I have discovered that mechanical penetration plays only a minor role in the sulphite process, being completed probably in less than one-half hour after thev solution is applied to the chips. The important factor in the sulphite process is diusion. vThe liquor at rst actually penetrates into the chips mechanically. The high vapor pressure of the sulphur dioxide displaces any air, the movement of the liquor being aided, perhaps, by such displacement. With the chips fairly well soaked, the phenomenon of digestion begins. The sulphur dioxide reacts with the lignin, thus:

RR'HC-CSO3HR"R" forming strong lignin sulphonic acid. With s'odium bisulphite as an illustration, the combined sulphur dioxide then neutralizes the strong acid as follows:

As can be seen from these reactions, the sulphurous -acid which reacted with -the lignin is regenerated so that the concentration of this reagent remains constant within the chip. A.Ils the bsulphite is consumed, more of it diffuses in the chip because oi' the difference of concentration.V and the neutralized lignin sulphonic acid diffuses outwardly for the same reason. Consequently, it is unnecessary in a sulphite operation to submerge the chips in liquor, as has been assumed heretofore, and it is therefore useless to dilute the solution of combined sulphurous acid to the point where an amount of solution required to submerge the chips is obtained. It is suflicient. as I have discovered, to keep the chips wet at their surface, so that additional combined suiphur dioxide may penetrate the chips by diffusion as the digestion proceeds.

'I'he amount of bsulphite consumed by wood in the ordinary sulphite cooking process is a fairly definite amount. I do not mean to convey the idea that it has any absolute value, for I do not believe that the lignin is completely sulphonated, or even that all of it is sulphonated to the same degree. However, for a given set of conditions, the consumption is fairly uniform. I have determined that western hemlock requires very close to 0.052 pound of combined sulphurous acid. per pound of oven dry wood. In practice. a slight excess is used. Generally the amount of combined sulphur dioxide required is between 0.05 and 0.06 pound per pound of dry wood, and the amount of metal bsulphite supplied in the form of solution should be in accordance with this requirement. Of course, under varied conditions with different woods', somewhat more or less combined sulphur dioxide will be required, and the figures are therefore illustrative vof the particular conditions here considered.

Since diffusion will occur just as readily if the chip surfaces are merely wetted and not completely immersed, I am able to use a much smaller volume of liquor which, containing initially the same amount of combined sulphur dioxide as is employed in the ordinary sulphite process, is actually much more concentrated, particularly at the beginning of the cook. The use of this concentrated solution of relatively small volume permits substantially complete consumption of the combined sulphur dioxide at the end of the cook.

In carrying out the invention in its preferred form, therefore, I employ a digester of the usual form which is substantially filled with wood chips or other cellulose material. To the digester I supply a relatively small quantity of liquor containing the required amount of combined sulphur dioxide. The amount supplied is such that only the chips in the lower section of the digester are actually submerged. The liquor is then circulated by means of a pump and is delivered to the top of the digester, where it is sprayed over the chips in such a way as to ensure thorough wetting of the surfaces thereof as the liquor drains downwardly through the mass of chips. For heating the contents of the digester, steam may be introduced directly into the circulating liquor or the liquor may be heated indirectly by means of a suitable heat exchanger. Sulphur dioxide may be introduced either before or after' complete diffusion of the combined sulphur dioxide has occurred. 'Ihe heating is conducted so as to gradually raise the temperature within the digester at a predetermined rate somewhat as in the ordinary sulphite process, although the heating may be more rapid and it is not necessary, as has been assumed, to defer attainment of the 2,192,289 c temperature of 110 for a period aslong as three hours.

One form of apparatus adapted for the practice ofthe invention is illustrated in the drawing, in which the digester 5 is of the usual form, having a closure 6 at its top and a blow valve 1 at its lower end. It is adapted to be filled with chips 8 to the level indicated at 9. The sulphite liquor is introduced in quantity to submerge the chips only to the level indicated at I0. The liquor is vwithdrawn through a pipe I I, delivered by a pump I2 through a pipe I3 to a'heat interchanger I4, and thence through a pipe- I5 to a distributor II at the top of the digester. 'I'hedistributor conveniently may be a perforated ring adapted to spray the liquor over the chips in the digester. Steam is supplied to the heat interchanger Il through a pipe I'I provided with a valve I8 and is withdrawn through a pipe I9 provided with a valve 20. Sulphur dioxide as required may be introduced through a pipe 2I provided with a valve 22. Thus, the liquor may be circulated continuously to maintain proper wetting of the chips without completely submerging them, and the heating may be regulated to control the cooking.

Sulphur dioxide may be introduced as desired.

As a specific example of the operation, the moisture content of the chips is first determined and the amount-of dry wood calculated. Enough bisulphite solution, such as sodium, magnesium, calcium or other bsulphite, is addedto aord 0.057 pound of combined sulphur dioxide per pound of dry wood at a concentration so thatthe total amount of liquor in the digester including the water in the wood is equal to approximately 24 gallons per hundred pounds of dry wood, as contrasted with 60 gallons of liquor per hundred pounds of dry wood as in the ordinary practice. The ratio of liquor to wood is," therefore, 2 to 1 instead of 5 to l as in the ordinary practice, assuming that the bsulphite solution has the same specific gravity as water. With wet hemlock wood, which is half water, 12 gallons of bisuphite 1 `solution give the desired ratio. In this case the percentage of combined sulphur dioxide in the bsulphite solution should be 5.7. If the ratio of bsulphite solution to dry material is 3 to l, the percentage of combined sulphur dioxide in the bsulphite solution should be 2.85, and as the ratio of bsulphite solution to dry material is varied between 2 to 1 and 3 to 1, the percentage of combined sulphur dioxide in the bsulphite solution should vary between about 5.7 and 2.85. After the liquor has been added, the' pump is started and thorough wetting of the chips is effected. Excess sulphur dioxide may be introduced in an amount depending upon the maximum pressure desired. Usually the amount required is from l5 to 25 pounds per hundred pounds of dry Wood. This sulphur dioxide may be introduced with the original bsulphite liquor either as a solution or as gas. The solution is then gradually heated by supplying steam to the heat interchanger through which the liquor is circulated. the rate of heating being that which will effect the most rapid pulping of the material with the minimum of loss. Heating is somewhat more rapid than in the ordinary sulphite process. Instead of adding sulphur dioxide before heating. the addition may be deferred until complete diffusion of the combined sulphur dioxide has taken place.

While the liquor ratio of 2 to 1 in place of the ordinary ratio of 5 to 1 is found to afford the most satisfactory operation, the invention is not 1I limited to this particular ratio, since obviously larger or smaller amounts of liquor may be used.

The essence of the invention is the use of an amount of liquor which does not submerge the chips, and the wetting of the chip surfaces by circulation of the liquor. .As a rule, ratios of bisulphite solution to the dry weight of the material of from 2 to 1 to 3 to 1 are most practical, but further variation is possible. Moreover, the invention is not limited to the use of combined sulphur dioxide asv bisulphite, since a mono-sulphite or a mixture of mono-sulphite and excess sulphur dioxide may be employed successfully in carrying out the invention.

'I'he use of a. ratio of liquor to dry wood as low as 2 to 1 or 3 to 1, while providing the amount of combined sulphur dioxide necessary for the l cook, obviously requiresa corresponding concentration of the combined sulphur dioxide in the liquor during the initial operation. The combined sulphur dioxide is substantially consumed by the cook, and the resulting waste liquor is similarly concentrated in the products resulting from the cook. 'I'hese products are recoverable at much lower expense, because it is no longer necessary to evaporate the large amount of water with which the waste products are diluted in the normal operation of the sulphite process as heretofore practised. Furthermore, the initial concentration of the combined sulphur dioxide material increases the rate of diffusion in the chips, permitting a more rapid rise of temperature and a consequent shortening of the cook without burning the chips. Furthermore, the use of a relatively small volume of liquor which submerges only a portion of the chips at the bottom of the digester, avoids the hydrostatic head due to great height of liquor in the digester, and avoids uneven gas concentration and uneven temperatures. Operation in accordance with the invention simplifies the recovery problem by avoiding excessive evaporation, and permits the recovery of heat which is lost in the ordinary sulphite process. 'I'he process as herein described gives a better yield of pulp, and a pulp having higher strength characteristics, with an expenditure of less sulphur dioxide.

Various changes may be made in the procedure as well as in the apparatus employed without departing from the invention or sacricing any of the advantages thereof.

I claim:

1. The improvement in the acid sulphite process for recovering cellulose from ligne-cellulosic materials which comprises submerxing a portion only of the material in a. concentrated solution of a bisulphite from the clau consisting of alkali metal bisulphites and magnesium bisulphite, the ratio of the weight of the liquor, including bisulphite solution and Water contained in the material, to the dry weight of the material being approximately between 2 to 1 and 3 to 1, the bisulphite liquor containing from about 5.7 to 2.85% of combined sulphur dioxide and sufcient free sulphur dioxide to complete the cook, and continuously wetting the surface of the overlying portion of the material with the liquor.

2. The'improvement in the acid sulphite process for recovering cellulose from ligno-cellulosic materials which comprises submerging a portion only of the material in a concentrated solution of a bisulphite from the class consisting of alkali metal bisulphites and magnesium bisulphite, the ratio of the weight of the liquor, including bisulphite solution and water contained in the material, to the dry weight of the material being approximately 2 to 1, the bisulphite liquor containing from about 5.7% to 2.85% of combined sulphur dioxide and suilicient free sulphur dioxide to complete the cook, and continuously wetting the surface of the overlying portion of the material with the liquor.

3. The improvement in the acid sulphite process for recovering cellulose from ligno-cellulosic materials which comprises submerglng a portion only of the material in a concentrated solution of a bisulphite from the class consisting of alkali metal bisulphites and magnesium bisulphite, the ratio of the weight of the liquor, including bisulphite solution and water contained in the material to the dry weight of the material being approximately between 2 to 1 and 3 to l, the bisulphite liquor containing from about 5.7% to 2.85% of combined sulphur dioxide, continuously wetting the surface of the overlying portion of the material with the liquor, and introducing free sulphurous acid during the cook after a period of diiusion.

4. 'I'he improvement in the acid sulphite process for recovering cellulose from ligno-cellulosic materials which comprises submerging a portion only of the material in a concentrated solution of a bisulphite from the class consisting ofv alkali metal bisulphites and magnesium bisulphite, the ratio o1' the weight of the liquor, including bisulphite solution and water contained in the material to the dry weight of the material being approximately 2 to 1, the bisulphite liquor containing from about 5.7% to 2.85% of combined sulphur dioxide,

continuously wetting the surface of the overlying GEORGE VICTOR PALMROBE. 

